CHAPTER FIVE

We can regard the current line of talks as a discussion of physiological adaptations to terrestrialization. Perhaps you can try to integrate several areas of physiology along those lines, including information about the integument, the respiratory system and the circulatory system. In our last meeting, we looked at the business of maintaining an optimal osmotic balance between the two major fluid compartments in insects, the intracellular compartment and the extracellular compartment. We saw that the osmotic balance was, in part, achieved by mechanisms that restrict and offset the unavoidable loss of water that must necessarily influence terrestrial organisms. I emphasize the water loss is unavoidable. In particular, we saw that the properties of the cuticle, control of spiracular openings and resorption of water from faeces are important mechanisms to control water loss. There are also a variety of mechanisms to replenish water supplies.

Today we want to continue to develop our appreciation of osmoregulation by observing that organisms must an optimal balance of water and of salt within the body. We develop the problem by considering some specific data. A brief look at the concentrations of various ions in an insect's regular food and in its hemolymph shows a couple of points. First, the compositions of the the food and of the hemolymph are remarkably different in many cases. Second, and more important, the differences are not uniform across the range of individual ions.

Here are examples. Larvae of the bot fly Gastrophilus are internal parasites of horses, mules and some larger wild animals. In this insect sodium is much higher in the hemolymph than in the food, while potassium is higher in the food than in the hemolymph. A different situation obtains in the stick insect Dixippus, which often eats Privet leaves. Privet comes from shrubs in the olive family. Here, sodium, potassium and calcium are much higher in the food while magnesium is much lower. Finally, in the Colorado potato beetle Leptinotarsa, there is very little sodium in the common food while potassium and calcium are much higher in the food than in the hemolymph. This table presents the specific data:

Now, if these salts were not somehow regulated, an insect would face the problem of osmotic, rather than water, imbalance between the two major fluid compartments every time it takes a meal. A clear example of this would be the large change in hemolymph calcium ions following a typical meal in the stick insect. So we want to ask: How are hemolymph ion concentrations regulated in the face of regular insult with each meal? Just imagine. One mechanism to regulate ion concentrations occurs at the level of the alimentary canal. The insect would simply limit the absorption of unnecessary salts. They would absorb only the required organic food material while unnecessary inorganics were passed though the insect and voided in due course. However, that is not the way hemolymph ion concentrations are regulated. In point of fact, dietary inorganic salts are readily absorbed across the epithelium of the alimentary canal and the ions therefore mix with the general pool of inorganic material in the extracellular compartment. The physiological problem, then, becomes one of moving ions into and out of the extracellular compartment in such as way as to sustain an optimal osmotic balance.

The most general expression is that secretion occurs in the Malpighian tubules and resorption takes place in the rectum. A generalized model of the alimentary canal shows the foregut, midgut and hindgut (Take a look). Within these familiar major regions are the Malpighian tubules, which most often join the alimentary tract in the midgut-hindgut junction, and the rectum, an enlarged portion of the hindgut featuring specialized epithelial cells. We want to appreciate the various sorts of movements of materials across the different regions of the alimentary tract. This is to be taken rather as a composite, and represents all sorts of movements, some of which do not occur in all insects. In due course we will see that water, inorganic ions, sugars and amino acids are mostly absorbed in the midgut.

With respect to ion concentrations, we want to see that water and ions are secreted into the lumen of the Malpighian tubules. These components are moved into the hindgut by mass flow, and then on to the rectum. Depending upon the individual needs of the insect, water and ions are reabsorbed by cells in the rectum and moved back into the hemolymph. We might get the impression that the same hemolymph constituents are moved in a sort of futile cycle, but we will see that the situation is otherwise.

Malpighian tubules are closed-ended tubules that arise from either the posterior part of the midgut or the anterior portion of the hindgut. The precise anatomical arrangement differs among insect groups. The walls of the tubules are one cell layer thick. The cells comprising the tubule walls stand on a basement membrane. The tubules from most insects are surrounded by muscles which move the tubules around in the hemocoel, and the tubules are supplied with numerous tracheae. Malpighian tubules of mosquitoes and some other insect species do not have muscles.

There are distinct histological and functional regions within the overall structures of Malpighian tubules from many, but not all, insects. The tubules have proximal and distal portions, proximal being closer to the attachment with the alimentary canal. The distal portion is functionally the secretory region, and features cells with honeycomb borders. By secretory we mean that water and ions are secreted from the hemolymph into the lumens of the tubules. The proximal region is composed of cells with brush borders; this is the absorptive region of Malpighian tubules.

We want to appreciate the cellular structure. The cells attach to the basement. Cells from the distal region feature a honeycomb border formed by cytoplasmic filaments. These filaments are rich in mitochondria, which supply the energy required for active transport processes. The cell membranes on the basal side are envaginated. All of these features are consistent with the secretory function of these distal cells. Cells from the proximal region of the tubule are different from the cells of the distal region in many insects. These cells feature a brush border. They are made up of more widely separated microvilli and have less complex envaginations of the basal-side membranes. They have more mitochondria in the basal side of the cell, thought to support their reabsorptive role in the tubule. Again, not all insects have Malpighian tubules of this heterogeneity. We also note that some insects, such as aphids, collembola and thrips have no Malpighian tubules or only small absorptive pads called papillae.

The Malpighian tubules of some insects have the distal ends adpressed to the sides of the rectum. The ends are surrounded by a membrane known as the perinephric membrane. The perinephric membrane forms a cavity between the rectum and the general hemolymph. This is known as a cryptonephretic arrangement. In the meal worm beetle Tenebrio this is thought to help absorb a maximum amount of water from feces.

A primary urine that is isosmotic to the hemolymph is formed in the Malpighian tubules. A general physiological observation is that water is rarely moved across membranes by itself. One way to remember this is to recall that ions can be moved by active pumping processes and water follows the locally formed gradient. In Malpighian tubules, fluid secretion is driven by active transport of potassium and chloride. In some blood feeders sodium is also actively transported. Potassium chloride (KCl) is usually the major solute in the lumenal fluid, and the concentrations of the ions are usually low compared to the hemolymph. The Malpighian tubule fluid makes its way to the alimentary canal by bulk movement aided by muscles. The fluid then moves through the hindgut to the rectum. In the rectum selective reabsorption of water, ions and various metabolites occurs by active transport processes. It is only in the rectum that the urine becomes strongly different in osmotic concentration from the hemolymph. Both tubular secretion and rectal absorption are under separate endocrine controls which together determine the amount and ionic composition of water that is excreted.

The secretion and absorption processes are summarized in the figure. This drawing is derived from the blood-sucking bug Rhodnius. This represents those insects with heterogeneous Malpighian tubules. In the distal region of the tubules potassium, chloride and sodium are pumped into the lumen by active processes; water follows the ion gradient into the lumen. Note that the figure does not show the movement of chloride. We will talk about uric acid formation and excretion later, but for now please note that uric acid also moves into the tubule. Resorption occurs in the proximal portion of the tubule, with potassium moved by active processes, and water following. After movement of urine and uric acid crystals to the rectum, more water can be resorbed, along with selected ions.

Some insects have homogeneous Malpighian tubules, such as the stick insect also represented in the figure. Secretion takes place all along the homogenous Malpighian tubules. Some movements are active processes, such as sodium and potassium pumps, while others are passive. There is no resorption until urine is moved into the rectum, where several ions, including amino acids are moved back into the hemolymph by active processes.

Again, here are data to make the point. This information comes from work on the locust, all in meq/liter:

These data show specific reabsorption of ions on an individual basis. The hemolymph is typically high in sodium, while potassium is the major urine ion in the tubules. These data indicate that sodium and chloride are selectively resorbed in the rectum. The osmotic concentration of the hemolymph and the urine are about the same, although of differing ionic compositions. The fluid components of the rectal contents are much more concentrated than the other fluids.

These data come from locusts fed with fresh water. By way of helping to prepare for any future exams that could crop up right out of a clear blue sky, consider what these data might look like if the locust had been fed salt water. There would be high sodium levels in the rectum, as an avenue to excrete excess sodium. Again, ions are individually regulated and transported. The key point is that the Malpighian tubules secrete water and salts in urine formation. Superimposed upon this urine formation mechanism is a resorption mechanism to facilitate the selective uptake from the urine of those specific things required to sustain the optimal osmotic balance between the intracellular and extracellular fluid compartments.

Consider these situations that an insect may confront::

- subjected to greater than normal desiccation

From a biological point of view, Malpighian tubules in some insects are adaptive in unusual functions. Larvae of the face fly Musca autumnalis have four Malpighian tubules, two anterior and two posterior tubules. Both pairs arise from a single junction between the mid- and hind-guts. The posterior set is found in close association with the midgut. The anterior set expands distally, and is filled with a granulated liquid. The posterior Malpighian tubules are regarded as ordinary secretory organs. The anterior tubules play a special role in the physiology of pupation. The puparium of this fly is hardened by deposition of calcium salts, rather than by the usual phenolic cross- linking typical of other insects. Dietary calcium salts, as well as other minerals, are stored in the anterior Malpighian tubules during the larval stages. At pupation, the calcium salts become dissolved, then transported from the tubules into the hemolymph. The calcium is directly transported from the hemolymph to the cuticle.

The Malpighian tubules of lacewing larvae undergo morphological changes before pupation. The modified tubules then produce silk for the pupal stage.